The biological potencies of a number of lipid fractions separated from endotoxins by acid hydrolysis, including the material known as lipid A, were determined in parallel with those of their parent endotoxins, employing bio-assays based on the following dose-related host responses: fever, resistance to infection, tumor damage, primary inflammation of skin, and lethality. Without exception, lipid fractions dispersed by detergents exerted less than 1 per cent of the biological activity of the potent endotoxins from which they were derived.

A study was made of the rate at which biologic activities diminished in relation to the release of bound lipid during progressive hydrolysis of Salmonella enteritidis endotoxin with dilute acid. Each of the five assays for endotoxin revealed that biological activity had been reduced to negligible proportions prior to any significant liberation from the endotoxin of water-insoluble firmly bound lipid. The major pharmacological activity of endotoxins, therefore, is acid-labile and cannot be accounted for in isolated lipids. This conclusion is also supported by the finding that lipids with activity similar to that of lipid A could be obtained by non-hydrolytic methods without diminishing the potency of the parent endotoxins.

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